Contact printing using high volume presses is commonly employed to print a large number of copies of an image. Contact printing presses utilize printing plates to apply colorants to a surface to form the image. The surface can form part of a receiver medium (e.g. paper) or can form part of an intermediate component adapted to transfer the colorant from its surface to the receiver medium (e.g. a blanket cylinder of a press). In either case, a colorant pattern is transferred to the receiver medium to form an image on the receiver medium.
Printing plates typically undergo various processes to render them in a suitable configuration for use in a printing press. For example, exposure processes are used to form images on an imageable surface of a printing plate that has been suitably treated so as to be sensitive to light or heat radiation. One type of exposure process employs film masks. The masks are typically formed by exposing highly sensitive film media using a laser printer known as an “image-setter.” The film media can be additionally developed to form the mask. The film mask is placed in area contact with a sensitized printing plate, which is in turn exposed through the mask. Printing plates exposed in this manner are typically referred to as “conventional printing plates.” Typical conventional lithographic printing plates are sensitive to radiation in the ultraviolet region of the light spectrum.
Another conventional method exposes printing plates directly through the use of a specialized printer typically referred to as a plate-setter. A plate-setter in combination with a controller that receives and conditions image data for use by the plate-setter is commonly known as a “computer-to-plate” or “CTP” system. CTP systems offer a substantial advantage over image-setters in that they eliminate film masks and associated process variations associated therewith. Printing plates imaged by CTP devices are typically referred to as “digital” printing plates. Digital printing plates can include photopolymer coatings (i.e. visible light plates) or thermo-sensitive coatings (i.e. thermal plates).
Many types of printing plates also undergo additional processing steps which can include chemical development. For example, chemical development steps are additionally required to amplify a difference between exposed and un-exposed areas. Other processing steps can include pre-heating and/or post heating steps. Once exposed or imaged, some printing plates undergo a pre-heating process so as to change the solubility of various regions of the printing plate in a subsequent chemical development process to achieve the desired differentiation between printable and non-printable areas. Post-baking of a chemically developed printing plate can be conducted to impart various desired characteristics to the printing plate, for example, increased plate life. Gumming processes can also be preformed to protect various surfaces of the printing plate from adverse environmental conditions. Further processing steps can include punching and bending procedures which can be used to impart various features on the printing plates to facilitate the mounting and registration of the printing plates on the press.
Various apparatus are employed to guide the printing plates through the various processes paths including various conveyors and gantries, which transport the plates between the various processing stations and present the plates at a given station with a positioning suitable for the particular processing associated with that station. Apparatus known as plate stackers are typically used to adjust an orientation of the printing plates at one or more points along their journey through the various processing steps. Plate stackers can be positioned after a chemical developer, heating or drying apparatus, punching device, etc. A sequence of printing plates is conveyed along a conveying path to various processing stations and the plates are collected and arranged in a stacked orientation by an apparatus such as plate stackers.
FIG. 1A schematically shows a conventional plate stacker 10. Plate stacker 10 is positioned proximate conveyer 15 which conveys a plurality of printing plates 20 along a conveying path 22. Printing plates 20 can include differently sized printing plates. Printing plates 20 are separated from one another along conveying path 22 by plate-to-plate separation distance S. In one form, plate stacker 10 includes a base 18 and a plurality of conveying members which facilitate a transfer of a printing plate 20 from conveyor 15. In this case, the conveying members consist of conveying belts 24, each of the conveying belts 24 having frictional attributes suitable for engaging the printing plates 20 supported thereon. Conveying belts 24 are driven by drive 35. Other plate stackers can employ other forms of conveying members including driven chains, rollers, etc. Plate stacker 10 also includes a plurality of pivoting members 26. In this case, pivoting members 26 are elongated members that are rotationally coupled to pivot 28 and are nested with conveying belts 24 as schematically shown by the plan view of plate stacker 10 in FIG. 1C.
As shown in FIG. 1A, conveying belts 24 transfers printing plate 20A from conveyor 15 until a leading edge portion 21 of printing plate 20A is detected by sensor 30, which stops the conveying belts 24. At this point, actuator 32 is operated to cause the pivoting members 26 to tilt or pivot about pivot 28 to engage printing plate 20A at a first position 34 and rotate it about a path to a second position 36 shown in FIG. 1B. In this case, second position 36 is a position in which the printing plate 20A is no longer supported by pivoting members 26 and is stacked with other printing plates 20 on plate stack 38. Printing plates 20 in plate stack 38 are shown separated from one another for clarity.
Printing plate 20A is moved from a first position 34 and a first orientation (i.e. inclination angle α) in which it is engaged by pivoting members 26 to second position 36 and orientation (i.e. inclination angle β) in which it ceases to be engaged by pivoting members 26. Upon stacking printing plate 20A on plate stack 38, pivoting members 26 are rotated back to their starting position. Pivoting members 26 can be driven to oscillate between their nested position and their plate stacking position.
A next printing plate 20B continues to move as pivoting members 26 undergo their oscillatory movement. Pivoting members 26 are required to return to their nested position prior to the transfer of a predetermined portion of printing plate 20B from conveyor 15. A return to the nested position is required since pivoting members 26 could strike printing plate 20B if transferred by conveying belts 24 prior to the return of pivoting members 26. Accordingly, the time taken by pivoting members 26 to engage a first printing plate 20 at first position 34, move the first printing plate 20 from first position 34 to second position 36, and then return back to first position 34 prior to the arrival of a second printing plate 20 can unduly limit the allowable plate-to-plate separation distance S. Consequently, plate stacker 10 can become a bottleneck in the processing of printing plates 20 by requiring a larger than desired plate-to-plate separation distance S between printing plates 20 along conveying path 22. Current exposure, chemical development and other processing systems have made, and continue to make, significant productivity improvements with respect to their specific processing and automation capabilities. These improvements can provide for a continuous stream of imaged and processed plates with continuously decreasing plate-to-plate separation requirements. These productivity improvements can be hindered by a plate stacker 10 that can not handle printing plates 20 provided at these higher rates.
There is a need for improved apparatus for re-orienting recording media such as printing plates along a conveying path. There is also a need for a plate stacker with enhanced plate stacking productivity capabilities.